The present invention generally relates to the treatment of industrial water systems to prevent corrosion damage. More specifically, the present invention relates to a method for treating an industrial water system to neutralize carbonic acid formed within the water system.
The primary considerations in the operation of industrial water systems, such as boiler systems, are maximizing energy efficiency and reliability, and eliminating unscheduled outages caused by water related problems. One economically attractive method is increasing the amount of condensate returned to the boiler as feedwater. Returned condensate, being condensed steam, is extremely pure and has a high heat content. Increased condensate return can improve boiler system economics through water and energy conservation.
As more condensate is returned, less make-up water is required, saving on water and make-up water treatment costs. The high purity allows for greater boiler cycles of concentration, thus reducing water and energy losses to blowdown. The high heat content (148 Btu/lb at 180.degree. F.) can provide substantial energy savings.
Corrosion in condensate systems can limit the quality and/or quantity of returned condensate. Iron and copper corrosion products can deposit on boiler heat transfer surfaces. This corrosion reduces heat transfer efficiency and could cause tube failure. In addition, corrosion caused by carbonic acid also damages components and piping in the condensate system.
Gases in the steam that dissolve in condensate to form a corrosive solution cause the condensate corrosion. Common gases found in condensate systems are oxygen, carbon dioxide and ammonia. Carbon dioxide and oxygen are most corrosive to ferrous metals, while oxygen and ammonia are extremely corrosive to copper and copper alloys. Either carbon dioxide or oxygen cause corrosion; however, the presence of both accelerates the corrosion rate significantly (10-40% faster than the sum of corrosion rates occurring from either gas alone).
Carbon dioxide can be found in steam due to the decomposition of bicarbonate, carbonate, or internal treatment chemicals in the boiler. The major source of carbon dioxide in steam is the break down of feed water bicarbonate and carbonate alkalinity in the boiler. At boiler temperatures and pressures, the following reactions occur: ##STR1## The first reaction proceeds 100% to completion. The second proceeds to approximately 80% completion. The liberated CO.sub.2 is carried with the steam into the condensate system. It becomes apparent that high alkalinity feed water will produce extremely corrosive condensate.
Carbon dioxide is not harmful until it dissolves in condensate. When the steam condenses, the carbon dioxide reacts with water to form corrosive carbonic acid. Since condensate is extremely pure, even small quantities of carbonic acid can significantly lower condensate pH and increase corrosivity. Corrosion rates increase with increasing temperatures. Since condensate is hot, this causes condensate to be even more aggressive to metal surfaces.
Reduction of alkalinity in the feed water decreases the carbon dioxide within the steam. This reduction of alkalinity can be accomplished by a mechanical means such as a well controlled lime softening program, dealkalization, demineralization, or degasification processes. Alternatively or in addition to the mechanical means, a chemical treatment program, such as neutralizing amines, may be used to neutralize carbonic acid in the condensate. When the neutralizing amines are used in addition to the pretreatment program, naturally, a better pretreatment with mechanical means results in a lower required amine dosage.
Organic amines are added to boiler feed water to volatilize into the stream and neutralize the carbonic acid in the condensate system. A variety of amines are used for this purpose. Blends of amines having different vapor/liquid partition coefficients are also used for better protection of the entire condensate system. Blends are advantageous because amines with smaller vapor/liquid partition coefficients will condense in the initial parts of the system and those with larger coefficients will protect the end of the system.
While the organic amines previously used in feed water systems have provided some protection against corrosion, such organic amines have several disadvantages. For instance, amines are difficult to handle since they are toxic, odorous, and flammable. Further, the copper corrosion problem becomes much more severe when the system contains oxygen, so the use of amine containing boiler blowdown for cooling tower makeup is normally undesirable.
In addition, neutralizing amines can and often do participate in the corrosion mechanism of copper and copper alloys by forming soluble complexes with the copper ions. Nathan, Corrosion Inhibitors, National Association of Corrosion Engineers, pp. 212-213 (1973). The source of the copper is usually the condenser and feedwater heater tubes within the boiler systems. Ammonia or substituted ammonium compounds attack the copper alloys and form copper complexes. Corrosion Basics, National Association of Corrosion Engineers, p. 169 (1984).
Therefore, a need exists for a new chemical treatment program that effectively neutralizes the carbonic acid formed within the feed water, while not participating in the corrosion mechanism.